光線維繫著我們對宇宙的感知。
有了光線，我們才能看到遙遠的星星，
探尋存在的起源。
不過，「光」到底是什麼？
簡單來說，組成光線的光子就是能量傳輸的最小單位。
光子是基本粒子，沒有嚴格的大小，
也不能分裂，只能產生和消亡。
光子也有「波粒二象性」，
也就是，在某種意義上，既是粒子又是波，
雖然這並不是真的。
另外，當我們說「光」的時候，一般都是指「可見光」。
其實可見光只佔有整個電磁頻譜的極小部分，
只是電磁輻射的少數典型。
不同的電磁輻射擁有
十分不同的波長和頻率。
伽馬射線的波長最短，因為其光子的能量最高。
多數伽馬射線只有不到十皮米的波長，
比氫原子小得多；
氫原子之於一分硬幣的大小，
相當於硬幣之於月亮。
可見光位於頻譜的中間，
波長介於 400 至 700 納米，
大概是細菌那麼大。
在頻譜的另一端，射頻電波的波長可以高達 100 公里；
而已知最大波長
可能超過十萬公里，
比地球大得多。
從物理的角度看，這些不同的電磁波都是一樣的——
它們都具有「波粒二象性」，也以光速「c」前進，
僅僅是頻率不同。
可見光有什麼特別之處嗎？
呃??沒有。
只是我們的眼睛正好能
感知這些頻率的電磁波。
不過，這也不完全是巧合
可見光是僅有的
在水中容易傳播的電磁波；
而最早的眼睛就是在水中進化出現的。
眼睛是一項偉大的發明，因為它接收的光線
不僅會與物質反應，還會為物質所改變，
所以可以幫助我們了解周圍的世界，
毫無延遲，
當然有利於動物的生存。
那麼，光是從哪裡來的呢？
當原子或分子由高能態轉變為低能態的時候
就會發出各種頻率的電磁波；
電磁波就是失去的能量。
從微觀角度看，當原子中的一個電子
從激發態跌至低能態並散發掉其多餘能量時
就會發出可見光。
同樣，光線也能擊中電子，被電子吸收，
令電子進入能量更高的狀態。
從宏觀角度看，變化的電子能量
產生一個震盪的磁場，
磁場又產生一個垂直的電場，
兩種場在空間中前進，
傳輸能量，
也傳送關於誕生地的訊息。
為什麼光是宇宙萬物中速度最快的呢？
從另一個角度問：
宇宙中最快的速度有多快？
就是「c」，真空中 299 792 458 米每秒，
每小時十億公里。
電磁輻射正好就是這麼快。
其實，任何零質量的粒子都只會以光速前進，沒有加速過程。
就是說，當光從蠟燭中出來的時候，
不會從零加速到光速；
產生的瞬間光就已經是光速前進。
那麼，為什麼光速「c」是有限的呢？
呃??沒人知。
宇宙就是這樣，
我們也不知道如何回答。
所以光是一種頻譜的一部份，
是一種像波一樣的基本粒子，
由兩種垂直的場推進，
以宇宙最高速度前進。
好了，就這樣。不過其他更加古怪的東西，
像以光速前進對時間的影響，
雙生子佯謬，量子力學之類的東西呢？
且看下次分解。
現在，為我們的眼睛歡呼吧：
它們接收穿梭時空的訊息，
讓我們「看見」萬物，從而思考我們的存在。
What is light?
Light, is the connection between us and the universe. Through light, we could experience distant stars and look back at the beginning of existence itself. But, what is light? In a nutshell Light, is the smallest quantity of energy that can be transported A photon, an elementary particle without a real size That can't be split, only created or destroyed. Light also has a wave-particle duality being kind of a particle and a wave at the same time (although this is a lie) Also when we say light, we actually mean visible light which is a tiny part of the electromagnetic spectrum: energy in form of electromagnetic radiation. Electromagnetic radiation consist of an enormous range of wavelengths and frequencies. Gamma rays have the smallest wave lengths because they are the highest energy photons. But most gamma rays are just under ten picometers, which is still way smaller than a hydrogen atom. For reference, a hydrogen atom compared to a cent is about as big as a cent compared to the Moon. Visible light is in the middle of the spectrum in a range of about 400 nanometers to 700 nanometers: about the size of a bacteria. On the other end of the spectrum, radio waves can be up to 100 kilometers in diameter. The biggest wave lengths we know exist can span from 10,000 kilometers to a baffling 100,000 kilometers, way larger than Earth. From a physics stand point, all these different waves are the same. They all have the wave-particle duality and travel at 'c', the speed of light, just at different frequencies. So what makes visible light special, then? Well... Absolutly nothing. We just happen to have evolved eyes, that are good at registering exactly this part of the electromagnetic spectrum. This is not a complete coincidence though. Visible light is the only set of electromagnetic radiation that propagates when in water, which happens to be where most eyes first evolved, millions of years ago. That was a smart move, because light not only interacts with matter, it's also altered by it and can be used to gather information about the world around us,with almost no delay. Which is arguably really helpful for survival. Okay, where does light come from? A vast range of electromagnetic waves are created when atoms or molecules drop from a higher state of energy to a lower one. They lose energy and emit it in the form of radiation. At the microscopic level, visible light is created when an electron within an atom in an excited state drops to a lower energy state and loses this excess energy. The same way, incoming light can elevate an electron to a higher state of energy by being absorbed by it. Macroscopically, the moving charge of the electron creates an oscillating magnetic field, Which creates an oscillating electric field perpendicular to it. These two fields move themselves through space, transferring energy from one place to another, carrying information about its place of creation with it. Why of all the things in the universe is light the fastest thing there is? Let's change the question: What is the fastest way to travel through space in the universe? It's c, exactly two hundred ninety-nine million, seven hundred ninety-two thousand, four hundred and fifty-eight metres per second in a vacuum, one billion kilometres per hour. Electromagnetic radiation just happens to move this fast. Any particle that has no mass travels at c, without acceleration or any in between. The light that has released from a candle does not speed up until it reaches light speed, At the very instant of its creation, its speed is c. So why is c, the speed of light, finite then? Well, nobody knows. Our universe is just built this way. We don't have the smart answer here. So light is part of a spectrum, an elementary particle that also behaves like a wave, Propelled by two perpendicular fields, travelling at the speed limit of the universe. Okay, that's nice and all, but what about the crazy stuff about travelling at the speed of light, and time, the twin paradox, quantum stuff, things like that? We would have to save them for another video. For now, let's be happy that we've evolved eyes that pick up waves of information permeating the universe. Making us see things and put our existence into perspective.
授課教師
陳永忠 ycchen@thu.edu.tw